a-(Si,Ge):H,F Alloys Prepared from SiH 4 and GeF 4
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a-(Si,Ge):H,F ALLOYS PREPARED FROM SiH 4 AND GeF 4 * D. SLOBODIN, S. ALJISHI, Y. OKADA&), D.-S. SHEN, V. CHU AND S. WAGNER Department of Electrical Engineering, Princeton University, Princeton NJ 08544
ABSTRACT The properties of a-(Si,Ge):H,F alloys prepared by glow discharge deposition from Sill4 and GeF 4 are described. The measured IR absorption spectra, sub-gap absorption spectra, dark conduction activation energies, carrier drift mobilities and deep trapping lifetimes of these alloys are similar to those of alloys prepared from SiF 4 , GeF 4, and H2 . However, they have over an order of magnitude lower photoconductivity over most of the composition range. Infrared absorption measurements indicate that these alloys have a fluorine content less than 1.5 at.% and a Si-H 2 content that increases with germanium concentration.
INTRODUCTION Considerable effort is being devoted to improving the electrical transport properties of hydrogenated amorphous silicon/germanium alloys, a-(Si,Ge):H.1'2'3'4 Fluorinated alloys prepared by glow discharge deposition from SiF 4 , GeF 4 and H2 are receiving particular attention because results to date suggest that these alloys may have properties superior to those of the alloys prepared solely from hydride source gases.5,6,7,8 However, a controversy exists as to whether 9 fluorine plays a beneficial role either in the deposition process or as a chemical constituent of the film. , 10 alloys, a-(Si,Ge):H,F, prepared from SiF 4 , GeF 4 , and H2 are fairly The fluorinated hydrogenated 5 6 7 8 well characterized. , , , ,11 In contrast, very little has been reported on the properties of alloys prepared by glow discharge deposition from a mixture of hydride and fluoride source gases such as Sill4 and GeF 4 . A preliminary study suggested that alloys prepared from Sill4 and GeF 4 have properties similar to alloys prepared from Sill4 and GeH 4.5 However, no specific data were reported. Further study of alloys prepared from SiH 4 and GeF 4 is clearly warranted in order to evaluate the quality of the material, to further the understanding of the role of fluorine in the alloys, and to gain insight into the connection between growth chemistry and film properties. In this paper, we describe the structural, optical, and electronic properties of a-(Si,Ge):H,F thin films prepared from Sill4 and GeF 4 (henceforth referred to as SHGF films). The properties of these films are compared with the properties of alloy films prepared from SiF 4 , GeF 4 and H2 (referred to as SFGFH films) in the same deposition system.
EXPERIMENTAL The alloy films were grown in a turbomolecular pumped glow discharge deposition station using a 2 DC proximity mode electrode configuration that has been described previously.1 The conditions for film a-Si:H from pure quality device prepare used to those to be chosen growth from Sill4 and GeF 4 were 2 Sill4 : discharge power density = 50 to 100 mW/cm , substrate temperature = 250 "C, pressure = 0.4 Torr, and total gas flow = 32 sccm. The substrate temperature was calibrated using
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